1. Field of the Invention
The present invention relates to hydraulic systems having a plurality of pumps and a plurality of independently controllable hydraulic actuators; and more particularly to controlling the plurality of pumps and allocating the resultant fluid flow to the plurality of hydraulic actuators.
2. Description of the Related Art
Hydraulic systems have at least one hydraulic pump that supplies pressurized fluid which is fed through control valves to drive several different hydraulic actuators. A hydraulic actuator is a device, such as a cylinder-piston arrangement or a hydraulic motor that converts the flow of hydraulic fluid into mechanical motion.
Because loads of different magnitudes act on the various hydraulic actuators, the hydraulic pressure required to operate each actuator can vary greatly at any point in time. On an earth excavator, for example, the hydraulic actuators that raise the boom typically require a relatively high pressure as compared to other actuators that curl the bucket or move the arm. Thus, when the operator is raising the boom at the same time the arm or bucket are also moving, a significant portion of the fluid flow from the pump will go to the lower pressure hydraulic actuators. Without some further compensation mechanism, this deprives the boom actuator of the necessary fluid required to operate as commanded. To maintain the proper flow sharing among all the actuators, the hydraulic systems use complex throttling mechanisms that add a pressure drop to the lower pressure functions and prevent them from consuming a disproportionately large amount of the fluid flow at times when multiple actuators are operating. Different equipment manufacturers use different throttling mechanisms. Some of these mechanisms use pressure compensators and a load sensing pump, while other ones use pilot pressure signals from the operator controls to create throttling losses for the low pressure functions. All these throttling losses generate heat and add inefficiency to the hydraulic system in order to enable the multifunction operation commanded by the machine operator.
It is desirable to avoid these intrinsic losses in efficiency and energy while still maintaining the multifunction performance desired by the operator.
The hydraulic system on many larger machines has multiple pumps that supply pressurized fluid for powering the various hydraulic actuators. One pump may be dedicated to supplying fluid to only selected actuators, while another pump furnishes fluid to the remaining actuators. A fixed assignment of hydraulic actuators to a given pump is inefficient when those hydraulic actuators are not consuming fluid and their pump is in a state of relative low use while a different pump for other hydraulic actuators is experiencing a heavy fluid demand. In other systems certain hydraulic actuators are powered by fluid from multiple pumps, in which case a mechanism is necessary for sharing the available fluid among those hydraulic actuators.
Therefore, it is desirable to allocate dynamically the fluid output from multiple pumps in an efficient manner, while recognizing the need for certain hydraulic actuators to have priority over other hydraulic actuators regarding the use of the available fluid.